Swimming Pool Designer & Expert Witness

Swimming Pool Design and Watershapes Consulting Expert Paolo Benedetti of Aquatic Technology Pool and Spa discusses concrete basics.

Concrete Strengths

The key to achieving proper concrete strengths is a direct result of the correct mix design and the proper handling of the plastic (fluid) cement.

The project engineer will specify a minimum concrete strength to be utilized.  There is nothing preventing the contractor or project owner from using stronger mix designs.  The added cost is negligible and the resulting performance and benefits are significant.

Mix Design

The project contractor will discuss the project requirements with the ready-mix plant's "Mix Master" when they place an order for the concrete.  They will discuss ultimate strength requirements, weather forecasts, traffic conditions, rate of placement (yards per hour), distance to pump and the pumping equipment.

The Mix Master will discuss the need for water reducers, plasticizers, air entrainment, set retarders or accelerators with the contractor.  The aggregate size should be the largest allowed by the project engineer.  The larger the aggregate the stronger the concrete will be.

Reduce the Water Content

Concrete is weakened by excess water. When water evaporates from concrete, it leaves behind a matrix of microscopic honeycombs.  These voids weaken the concrete.  They also allow ground or rain water to permeated into the concrete.  This can result in corrosion of the reinforcement steel and eventual project failure.

Only that amount of water that is necessary to hydrate the cement particles is required.  To increase the slump (liquidity) of the mix design, water reducers & plasticizers should be used.  Think of these additives a being able to s-t-r-e-t-c-h the water.  The resultant strength will be significant, though the initial concrete will be liquid, workable and pumpable.

A reputable ready-mix plant will even know the moisture content of their sand and aggregates.  They will subtract from the water to be added to the mix, the water inherent in the aggregates.  A load tag will accompany the delivery truck.  The assay of the contents of the truck should be specified, along with the load time and starting count on the truck's rotating drum.

My ready-mix supplier even provides the assay of the aggregate in their mix.  The load tag indicates the psi compressive strength of the rocks in the mix.  

It makes absolutely NO SENSE to use concrete that contains rocks weaker than your objective.  But, there are many ready-mix suppliers who use weak and inferior aggregates (e.g. quartz ).  If they don't volunteer this information - ask.  They are required by the building codes to tell you.

Achieving High Strength Concrete

Higher strength concretes are denser and more durable.  This is achieved by "tightening" the matrix of the concrete.  The increased cost of a higher "sack" mix is minimal (e.g. a 4 sack mix vs a 6 sack mix).  A "sack" is equivalent to 94# of cement powder (about 1 cubic foot).

There are many means to increase concrete's strength.  Many of the methods are used in conjunction with each other:

• Reduce the water content
• Increase the aggregate size from 3/8" to 3/4"
• Add pozzolans (Supplementary Cementitious Materials - SCM)
• Utilize crystalline or water proof admixes 
• Corrosion inhibitors 
• Air entrainment (freeze-thaw conditions)
• Metakaolis (help reduce efflorescence)
• Integral color pigments

Some additives are not compatible with others, so be sure to read the labels and warnings (e.g. fly ash is not compatible with some waterproof admixes).

Integrity of the Mix Design

It is the responsibility of the contractor to ensure that the mix design is not adultered by "well meaning" individuals.  Concrete pump operators and ready-mix truck drivers are the worst culprits.  When the truck arrives, the pump operator tells the driver to add water to the mix.  Before the concrete is even out of the truck, the strength is being compromised.

After the truck arrives, if you hear the engine revving and the drum spinning rapidly they are adding and mixing additional water.

Proper Handling

The concrete should not be allowed to sit in a truck, waiting for prior trucks to be unloaded.  The concrete may achieve it's final set while in the truck, rendering it useless (unless set retarders were added).

The truck must achieve a minimum number of rotations prior to off loading.  The dry ingredients are added to the truck at the ready-mix plant.  It is the rotation of the drum and vanes that actually mix the concrete.

Concrete that is to be placed into tall forms, caisson holes, deep trenches or footings should be placed from the bottom up.  It should not be pumped in from the top and allowed to fall to the bottom.  This separates out the aggregate and compromises the strength.  It also creates large voids and air pockets in the finished product.

Proper consolidation (vibration) must also be preformed.  Excessive vibration will also cause the aggregate to settle to the bottom.  The proper mix design can virtually eliminate the need for consolidation.

During

Concrete does not "dry," it cures.  As the water hydrates the components of the concrete mix, crystalline structures are formed.  These are what give the concrete it's density.  Controlling the rate of water evaporation is critical.  The goal to to have the water leave the concrete as slowly as possible.  This is achieved through proper curing. 

Coverings of burlap, carpet or plastic are the most rudimentary.  This method cause surface discoloration due to trapped moisture under the covering. They are also susceptible to being moved or blown out of position.

Water curing is performed by maintaining the moisture content of the concrete by frequent sprinkling, spraying or flooding with water.  This is not always practical as there may be water restrictions, drought conditions, lack of personnel 24/7, the project is too large or the site may become excessively muddy.

Chemical curing compounds are chemicals that are sprayed onto the finished concrete after it becomes stiff, but is still damp in appearance.  Some chemically penetrate the concrete while others form a surface film.  These are the method preferred by most State Highway Departments because they are the most infallible.

Care must be taken in selecting the proper curing solution.  Some of them create a "bond breaker" that inhibits the bonding of subsequent surfaces.  Subsequent decorative finishes of plaster, stucco, paint or stone work may not bond to the concrete.  

Then why do they even make those types?  Contractors who pour roadways, bridges, sidewalks and other high traffic surfaces are the largest consumers of these type of curing compounds.  They are also the most inexpensive.  Most contain some type of waxy substance.  The curing film often dries white (or has a dye), to indicate areas of coverage and proper application.  Over time, the film will eventually wear away from traffic, abrasion and weather.  

Remember, they are concerned with lifetime performance of their structure (as should you!)
 

Paolo Benedetti - Aquatic Artist 
"Creating water as art."™ 
Aquatic Technology Pool and Spa 
©www.aquatictechnology.com

Cracking Concrete Pool Decks

Cracking Concrete Pool Decks

Swimming pool concrete deck expert witness Paolo Benedetti, discusses improper slab thickness and the lack of proper reinforcement steel placement.

The Concrete Pool decks are cracking
There is no code that requires reinforcing steel within non-structural concrete "slabs on grade."  However, if any reinforcement is used, then the codes are very explicit as to where and how it is to be placed.

There ought to be a law!
The Uniform Building Code makes direct reference to the standards established by the American Concrete Institute.  For slabs poured on grade there must be a minimum of 3 inches of concrete between the reinforcement and the earth.  It also states that there must be a minimum of 1.5 inches of concrete on top of the reinforcement (side exposed to the weather).

ACI Standards require a Minimum Slab Thickness 
Because the ACI specifies the amount of concrete around the reinforcement, it de facto creates a standard for minimum slab thicknesses.

If #4 reinforcement steel is used to reinforce a slab on grade, then the slab must be a total thickness of no less than 5.5 inches.

1.5 inches of coverage over the steel
1 inch of steel (#4 bars are 0.5 inch thick, where two #4 bars cross there is 1" of steel)
3 inches below the reinforcement steel
1.5 + 1 + 3 = 5.5" minimum slab thickness (when using #4 bars).

2x4 Formwork is INCORRECT!
2x4's are actually 3.5" wide.  Therefore there is no means for a concrete contractor to pour concrete slabs of sufficient thickness if they utilize 2x4's as forms!

2x6's are 5.5" wide.  They are the MINIMUM sized lumber that should be used when placing concrete slabs.

The ACI standard applies whenever reinforcement is used within the concrete.  This even means when they use #10 wire mesh, that they must support the reinforcement so that it is placed in the proper location within the slab.

"Hooking" the wire mesh & lifting it up into the wet concrete will not place the mesh in the proper location. 

This is why most concrete slabs are cracking, too thin, incorrectly formed and with the steel in the wrong location! 


Paolo Benedetti - Aquatic Artist
"Creating water as art."™
 Aquatic Technology Pool and Spa
©www.aquatictechnology.com

There's NO such thing as WATERPROOF CONCRETE - or is there?

Internationally renown Los Angeles swimming pool designer & builder Paolo Benedetti of Aquatic Technology Pool & Spa, lecturer, educator, published author and industry expert, discusses the truths behind claims of "waterproof concrete."

Lately there has been a lot of misinformation provided about "watertight" or "waterproof" concrete. This misinformation is the result of people confusing the two terms PERMEABILITY and WATERPROOF.

Let's first outline some basic scientific facts (per the American Concrete Institute - ACI):
1. CONCRETE IS PERMEABLE (ACI),

THEREFORE,

2. CONCRETE CANNOT BE WATERPROOF
(without taking additional measures such as special admixes or surface coatings - ACI).

3. THE DENSITY OF THE PLACED CONCRETE AND (the original) WATER CONTENT OF THE MIX DESIGN - ONLY AFFECT IT'S PERMEABILITY.

False Claims? or Just Confused?
To claim that a dense concrete vessel is watertight or waterproof is a scientifically false statement. It may in fact have a low permeability (the ability of the water to pass through the concrete), but it will still have some measurable level of permeability. You may be able to fill it, and it might not appear to lose any water, but it is still permeable - and therefore losing some water. The mere fact that water can pass into the concrete, illustrates that it cannot be WATERTIGHT or WATERPROOF. It is physically impossible (without the assistance of admixes), regardless how dense the concrete is.

The higher the level of hydrostatic pressures upon the surface of the concrete, the greater the distance the water will permeate the concrete. Given concrete of the same density (PSI,MPA) and therefore the same level of permeability, here is an example of how permeability can affect "water tightness" of a concrete structure:
a. Under water at the upper elevations of a large concrete dam, there is barely any hydrostatic pressure upon the surface of the concrete. The water will penetrate and saturate the concrete only to a certain depth.
b. At the base of the dam, where there exists extreme levels of hydrostatic pressure, the water will penetrate a greater distance into the concrete wall. This occurs even when the concrete has the same density as the upper elevations.

In the case of the Hoover dam, water is actually seeping through the concrete walls at the lower elevations! This is because ALL concrete is permeable. The hydrostatic pressure is actually driving the water through the permeable matrix of the concrete.

Increase the Density
Increasing the density of the concrete can only reduce the permeability of the concrete, but it cannot eliminate it. Microscopically, there are a multitude of passages through the concrete. There are small cracks around the aggregates. There are small fissures in the concrete from the shrinking that occurs during it's curing stage. There are voids where the water in the original mix once was.

Methods to increase the density of concrete, and thereby reduce it's permeability are achieved by:
1. reducing the water in the original mix (utilize super-plasticizers)
2. increasing the "fines" in the original mix (silica fume particles are 100 times finer than cement particles)
3. practicing approved curing methods
4. proper placement and compaction of the wet concrete
5. employing specialty "waterproof" (densifying) admixes

Silica Fume
Because of their small size in relation to cement particles, the silica fume particles can fill in tiny voids that occur between the cement & aggregate particles. The silica fume particles will react with the free lime that is released during cement hydration. The end products are calcium silicate hydrates (CSH). These CSH particles replace the weaker lime that is normally found in concrete. In field use of silica fume as a component of a mix design will reduce permeability by 20 times, over a mix design without silica fume.

"Waterproof" Admixes
The use of "waterproof" admixes (e.g. Xypex, Kryton, etc.) merely make the concrete so dense, that it becomes for all intensive purposes, impermeable. The shell is technically waterproof, because the concrete is impermeable. The presence of water with these admixes, actually promote additional crystalline growth - sealing the permeable microscopic voids, a term called "hydroscopic self-healing." Mind you, this is occurring on a microscopic level, so these crystalline structures will not heal structural or shrinkage cracks. However, poorly placed, improperly cured, or too much water in the concrete will defeat any benefits of these admixes.

Untreated 1. CONCRETE (UNTREATED) A control sample of concrete was sheared through at 50 mm below the top surface. The sheared face shows some of the by-products of cement hydration with which Xypex reacts. Precipitated calcium hydroxide together with cubic and rhombic particles are visible. (courtesy of www.Xypex.com)

Initiation 2. XYPEX CRYSTALLIZATION (INITIATION) Taken at 50 mm within a Xypex-treated concrete sample, this photograph shows the initiation of the Xypex crystalline reaction after Xypex Concentrate was applied to the surface. (courtesy of www.Xypex.com)

Mature 3. XYPEX CRYSTALLIZATION (MATURE) This photo was taken 26 days after the application of Xypex Concentrate at a depth of 50 mm into the concrete sample. A dense, fully developed crystalline structure has formed within the capillary tracts of the concrete to completely block the flow of water. (courtesy of www.Xypex.com)

Permeability Testing
The problem with the widely utilized Rapid Chloride Permeability Test, is that it does not measure the depth of the chloride penetration or how rapidly the chloride ions reached a specific depth. An accurate measurement of permeability must be able to measure BOTH the depth and rate of penetration.

Alternate tests that provide these measurements do not take into account any atmospheric or environmental conditions that may increase the penetration (hydrostatic pressure), as these tests were created by highway departments to measure the quality of in-place concrete. The surface of a concrete slab, overpass, or guard railing has virtually ZERO hydrostatic pressure upon it.... only the thickness of the rainfall!

Because ALL concrete is permeable to some degree, we should all be striving to achieve maximum density in our placed concretes. This lessens the permeability and the chances of reinforcement corrosion - but, it does not eliminate the risk.

Waterproofing / Watertight
The ONLY means to create a WATERPROOF or WATERTIGHT vessel, is to eliminate permeability - which by concrete's nature is scientifically impossible. If you are willing to forgo additional waterproofing measures, then you are also accepting an unknown level of permeability into your concrete structures. The level of permeability may be minuscule, but it is an UNKNOWN that I am not willing to accept.

To actually claim that a vessel is WATERPROOF or WATERTIGHT, without the additional use of waterproof barriers is false. The concrete is still permeable, maybe it is less permeable than vessels built by others, but it is not water proof or watertight! The only means to achieve a waterproof or watertight vessel is to 100% eliminate the permeability.

The only way to stop the permeability, is to keep the water away from the concrete. This is where waterproofing measures come into play. Curing sprays do not make a vessel waterproof - they only help reduce the permeability, so do not buy into the "waterproof" sales pitch.

Because waterproofing cannot be applied to the reverse of a concrete vessel shot against earth, it is imperative that proper placement, compaction, mix design and curing procedures be followed. The addition of waterproofing admixes will reduce the permeability of the concrete from the reverse side, protecting the reinforcing steel and thereby increasing the structure's lifespan.

Be sure that you use the correct terms:

INCREASED DENSITY REDUCES PERMEABILITY AND INCREASES DURABILITY (but it is not waterproof).

WATERPROOF MEANS THAT WATER CANNOT / DOES NOT ENTER (i.e. PERMEATE) THE CONCRETE.

(Scanning electron microscope images & descriptions used with permissions and courtesy of www.Xypex.com)

Paolo Benedetti - Aquatic Artist"Creating water as art."™Aquatic Technology Pool & Spa©www.aquatictechnology.com